Thermostat for electric ovens



Dec. 26, 1933. E, L, FONSECA 1,941,080

THERMOS'TAT FOR ELECTRIC OVENS Filed May 11, 1931 INVENTOR ATTORNEY Patented Dec. 26, 1933 UNITED STATES PATENT OFFICE The Wilcolator Company, Newark, N.

corporation of Delaware Application May 11, 1931. Serial No. 536,509

14 Claims.

This invention relates to thermostatic control devices for controlling temperatures in electrically heated apparatus such as electric stoves or ovens, and is concerned more particularly with a thermostatic controller of novel construction for operating a switch by which the current supply for heating units is controlled so as to maintain a heating chamber at a selected temperature.

In most thermostatically controlled electric switches with which I am familiar, the mechanism is made delicate and sensitive to afford control of the temperature within the desired limits and as a consequence these devices have not been sufficiently rugged or capable of sufliciently rapid operation to permit direct control of the full load current. Consequently it has been customary to use a relay which is controlled by the thermostatically operated switch and which in a turn controls the full load switch.

I am aware that it has also been proposed to employ mercury switches in thermostatic control devices and the present invention is directed to an improvement on that type of device and 26 differs from the prior constructions in numerous particulars. The device of the present invention includes an enclosed mercury switch which is tilted to make and break the circuits to the heating elements, and includes means for giving a 10 continuous indication of the temperature, the heat-responsive elements acting as a thermometer as well as serving as the operating means for the switch. In order that the thermostat may perform this dual function, provision is made in the 85 device to permit it to be set for different temperatures without changing the calibration of the thermostat and thereby destroying its utility as a thermometer. To this end, the setting of the device involves a shifting of the switch bodily without imparting angular movement thereto,

and the switch may have a normal bias to one position so that it may be sensitive in operation and this bias is maintained without change throughout the setting of the device for different a temperatures.

The mechanism includes numerous features of novelty in addition to those above mentioned which will be made clear in the detailed description to follow.

For a better understanding of the invention, reference may be had to the accompanying drawing, in which Figure 1 is a longitudinal section of the thermostatic switch of this invention;

Figure 2 is a face view of the same with the cover removed;

Figure 3 is a transverse section as seen along the line 3-3 of Figure 1; V

Figure 4 is a plan view of the switch with the on cover broken away to expose the interior thereof; and

Figure 5 is a diagram of the various positions of the operating elements.

In the drawing, the device is illustrated as 5 installed in the wall 10 of the heated chamber through an opening 11, in which is inserted the sleeve 12. The sleeve 12 is secured at one end to the base plate 13 of the device and is threaded at the other end for the reception of the cage, 14, which may be drawn up so that the base plate 13 is clamped tightly against the wall 10.

Within the sleeve 12 is a tube 15, which carries at one end a disc 16 fitting loosely in sleeve 12 and having a radial member 16 lying behind base plate 13 and adjustably secured thereto by a screw 17 passing through a curved slot 18 in the base plate. The member 16 passes through an opening in sleeve 12 and may be moved to rotate disc 16 within the sleeve. One end of a helical go thermostatic strip 17', which responds to changes in temperature by coiling and uncoiling, is secured to the tube 15 near one end and the other end of the strip 17 is secured to the end of a rod or shaft 18','which extends through tube 15 and 5 carries on its outer end a bushing 19 which extends through an opening in the baseplate 13 in alignmentwith the end of the tube.

A bell crank 20 is secured to the face of bushing 19 by soldering, or the like, and one leg 20a of the crank terminates in a pointer 21 movable over curved scale 22. The scale 22 is secured to the base plate 13 in any convenient way and is inscribed with appropriate legends or numbers indicating temperatures. Journalled on bushing 19 between crank 20 and a washer 23 on the outer end of rod 18', is a lever 25, having lug 24 engaging the edge of the other leg 20b of the crank, the end 26 of the lever being connected by a coil spring 2'7 to a lug 28 on leg 20a of the crank. The spring 27 urges lever 25 in a counter-clockwise direction so as to keep the lug 24 in constant engagement with the corresponding edge ofthe leg 20b for a purpose which will be described later.

Supported on posts 29 mounted on base plate 5 13 and secured to these rods byscrews 30 is a frame 31, preferably of sheet metal, having two lugs 32 (Fig. 2). Parallel links 33 and 34 are pivoted on the lugs and connected pivotally at their upper ends to the plate 35. The spacing between 54, such as bakelite,

the pivots of links 33 and 34 is the same and the arms are of the same length, thus forming a pantograph, whereby any movement of links 33 and 34 results in a bodily shifting of plate 35 without swinging thereof. Link 33 is provided with a pointer 36 which cooperates with the scale 22, pointers 21 and 36 lying at opposite sides of the scale. The pivot of link 33 on its lug 32 is concentric with the rod 18, so that both pointers 21 and 36 swing about the same axis.

A frame 37 (Fig. 3) is pivoted on a pin 38 on plate 35 and the movement of frame 37 is limited by means of a pin 39 on the frame which extends into a hole 40 in plate 35 (Fig. 2). Two clips 41 are suitably secured to the under surface of frame 37 and carry a sealed mercury contact switch 42. The switch consists of a glass tube through the wall of which are sealed contact terminals 43 and 44 and the tube contains mercury 45 which connects the contacts in one position ofthe switch and opens the connection in another position. Wires 46 and 47 connect contact terminals 43 and 44, respectively, with terminal plates 48 and 49, respectively, suitably mounted on an insulating block 50, such as bakelite, on the frame 37. The ends of the terminal plates 48 and 49 are turned downwardly (Fig. 1) and lie adjacent the axis of the pivot pin 38 about which the frame 37 pivots, so that their presence does not interfere with the movement of frame 37 in response to a slight impulse. Insulated wires 51 connect the terminal plates 48 and 49 to binding posts 52 and 53, respectively, the posts being mounted on an insulating block secured by cleats 55 to the base plate 13. Upon the rear surface of frame 37 and displaced eccentrically from pivot pin 38,

is a pin or detent 56, arranged to lie in the path of movement of the lever 24.

A cover plate 57 is secured over the flanged edge of base plate 13 or is otherwise mounted in position to enclose the apparatus. Journalled on a pin 58, secured to the cover plate 57 in line with the lower pivot point of link 33, is a knob 59 which is frictionally held in any adjusted position by spring 60 interposed between the knob 59. and the cover plate 57. The rear surface of the knob 59 is provided with a socket for receiving the outer end of a pin 61, whose inner end is secured to link 33, the pin 61 passing through a curved slot in cover plate 57. Rotation of knob 59 is transmitted by pin 61 to the pantograph, and results in switch 42 being shifted bodily in a horizontal position with angular movement.

In the use of the device, for example with an electric stove, one side of the full load current supply to the electric resistance heating units of the oven is connected to terminals 52 and 53, so that the mercury switch 42 is directly in the heating circuit and takes full load current. By using a mercury switch, the device is capable of making and breaking full load current having the voltage normally employed in heating electric stoves without being damaged.

In setting the device for a selected temperature, the operator manipulates knob 59 until pointer 36 indicates the temperature on scale 22. Current now flows through a circuit including the line, terminal 52, wire 51, terminal plate 48, wire 46, contact 43, mercury 45, contact 44. wire 47, terminal plate 49, wire 51, terminal 53 and through a conductor connecting terminal 53 with the stove resistors. In the normal position of the switch 42, mercury 45 connects terminals 43 and 44, the switch 42 being tilted downwardly to the right' (Figs. 2 and 3). In that position of the switch, pin 39 engages the side of the aperture 40, and since the center of gravity of the frame 37 and the parts which it supports is slightly to the right of its pivot 38, the switch 42 is normally biased to closed position. The bias of the switch mounting is slight and little force is required to move the frame 37 to a position in which the mercury 45 flows to break the connection between terminals 43 and 44.

As the oven heats up, the pointer 21 is moved by thermostat 17' to indicate the instantaneous temperatures of the oven and when the temperature for which the device is set is reached, lever 25 which moves with the leg 20b engages detent 56 and moves tilting plate 37 about its pivot 38, so that the mercury 45 in switch 42 breaks the circuit.

As illustrated diagrammatically in Figure 5, the displacement of dete'nt 56 above pivot 38 of the tilting plate 37 compensates for the width of the lever 25 and insures that both pointers 21 and 36 will line up exactly when the predetermined temperature is attained. This avoids errors due to angularity of the parts. Also, the displacement of detent 56 with respect to pivot 38 provides a crank arm, whereby the tilting of the switch may be accomplished by a very small pressure of lever 25 on the detent. As soon as the oven cools down slightly after the circuit to its heating units is broken, the thermostat 17 withdraws lever 25 from detent 56, and the overbalanced switch again drops into closed position and the circuit is remade. These actions are repeated from time to time, so that the oven is j maintained at a substantially constant temperature.

It sometimes happens with a device of this sort that the setting is altered from a high temperature to a low one at a time when the thermostat has been heated to a high temperature. In the present device, the lowering of the setting involves shifting of frame 37 to the right (Fig. 2) and movement of detent 56 toward lever 25. If

this lever were rigidly attached to the shaft 18',

the movement of the detent would cause the shaft to rotate and this would put a considerable strain on the thermostatic coil due to its heated condition. However, lever 25 is operatively connected to crank 20 and shaft 18 through the spring 27 1 and if a movement of detent 56 is opposed by lever 25, the resistance of the spring is overcome and no strain can be imposed on the thermostatic element greater than is required to stretch the spring. This permits resetting the device under all conditions without liability of damage to the thermostatic element, or substantial destruction of the indicated oven temperature.

In order to calibrate the instrument, screw 17 is loosened and member 16' moved to vary the degree of twist of thermostat 17, and thus alter its operating characteristics. This calibration may be done while the device is mounted on the oven and without removal of any part except the cover 57. This arrangement is particularly valuable for calibrating the device either by the cold check or hot check methods, and permits exact calibration of each device as installed without the necessity of careful andexpensive calibration at the factory.

Itwill be seen that the new thermostatic switch provides many important advantages over those heretofore known, in that it eliminates relays and special wiring, is self-contained, insulated against short-circuiting and arcing, and has the additional advantages of continuously indicating the temperature of the heating chamber in which it is mounted.

I claim:

1. A thermostatic control device which comprises the combination of an element responsive to temperature variations, an indicator actuated by said element for continuously indicating temperatures, a mercury switch actuated by said element at a predetermined temperature, and means to vary the relative positions of said indicator and switch without tilting the latter for selecting said temperature. I

2. A thermostatic control device which comprises the combination of an element responsive to temperature variations, a mercury switch tilted by said element at a predetermined tem-- perature, and means for selecting said temperature by bodily shifting said switch without imparting angular movement thereto.

3. A thermostatic control device which comprises the combination of an element responsive to temperature variations, a mercury switch mounted for tilting movement and biased toward vione position, means operated by said element and effective at a predetermined temperature for tilting said switch to a second position, and means for varying the relation between said switchand means without tilting said switch.

4. A thermostatic control device which comprises the combination'of an element responsive to temperature variations, control means for the source of heat developing the temperatures to which said element is exposed including a mercury switch, a member for actuating said switch and operatively connected to said element at a predetermined temperature only, and means for selecting the temperature at which said control means is to be actuated, said control means being moved to different positions by said selecting means without tilting said switch.

5. A thermostatic control device which comprises the combination of an element responsive to temperature variations, an indicating member moved by said element to provide a continuous indication of the temperature to which said element is exposed, control means for the source of heat developing the temperatures to which said element is exposed, said control meansincluding a mercury switch pivotally mounted, and a support for said switch, means for actuating said control means when a predetermined temperature is reached, and means for selecting said temperature, said means operating to produce a bodily shifting of said switch without angular movement.

6. A thermostatic control device which comprises the combination of an element responsive to temperature variations, an indicating member moved by said element to provide a continuous indication of the temperatures to which said element is exposed, control means for the source of heat developing the temperature to which said element is exposed, said control means including a pivotally mounted mercury switch and a pantographic support therefor, means for actuating said control means when a selected temperature is reached, and means for selecting said temperature, said means operating on said support to cause a shifting of said switch without angular movement thereof.

'7. A thermostatic control device which comprises the combination of an element responsive to temperature variations, an indicating member moved by said element to provide a continuous pivot when a selected temperature is reached, and

means for selecting said temperature, said means operating to cause a shifting of said switch without angular movement thereof.

8. A thermostatic control device comprising the combination of a thermostatic element, control means for the source of heat developing the temperature to which the element is exposed, said means including a circuit closer operated by being moved angularly, means for imparting angular movement to said part when a selected temperature is reached to actuate said control means, and means to bodily move said circuit closer without angularly displacing the same for selecting said temperature.

9. A thermostatic control device comprising the combination of a thermostatic element, an indicator operated by said element to give a continuous indication of the temperature to which said element is exposed, control means for the source of heat developing the temperature to which the element is exposed, said means including a part operated by being moved angularly, means for imparting angular movement to said part when a selected temperature is reached to actuate said control means, and means for selecting said temperature, said selecting means operating to move said part bodily but not angularly.

10. A thermostatic control device comprising the combination of a thermostatic element, an indicating arm moved by said element to give a continuous indication of temperature to which the element is exposed, control means for the source of heat developing the temperature to which the element is exposed, said means including a part operated by being moved angularly, said part being actuated through said indicating arm, and means for selecting the temperature at which said control means is actuated, said means operating to shift the position of said part without imparting angular movement thereto.

11. A thermostatic control device comprising the combination of a thermostatic element, an indicating arm moved by said element to give a continuous indication of temperature to which the the element is exposed, a pivotally mounted mercury switch, a support for said switch mounted on a parallel link structure, said switch being movable on its pivot by said arm when a predetermined temperature is reached, and means operating on said structure to effect a shift of said support without moving said switch angu-- larly, said shifting varying the temperature at which said switch is moved by said arm.

13. A thermostatic control device comprising the combination of a helical thermostatic element, a shaft rotated by said element as --the latter is exposed to varying temperatures, a crank arm fast on said shaft and giving a. continuous indication of the temperatures to which said element is exposed, a mercury switch pivotally mounted, a support for said switch, said switch being tilted by said arm when a selected temperature is reached, and means for selecting said temperature including means for moving said support to shift said switchv without tilting it.

14. A thermostatic control device comprising the combination of a helical thermostatic element, a shaft rotated by said element as the latter is exposed to varying temperatures, a crank arm fast on said shaft and giving a, continuous indication of the temperatures to which said element is exposed, a mercury switch pivotally mounted, a support for said switch, an arm loose on the shaft and spring pressed to move with said crank arm, said loose arm causing a tilting of said switch when said element has been exposed to a selected temperature, means for selecting said temperature including means for moving said support to shift said switch without tilting it, and means for calibrating said element.

EDWARD L. FONSECA. 

